The present invention broadly relates to a method of green balling iron-bearing fume, which is recovered during oxygen steelmaking operations, to form strong, stable balls suitable for charging into steelmaking furnaces. More particularly, the present invention relates to an economical method of forming dry-collected iron-bearing fume, which contains lime (CaO) and magnesia (MgO) in a combined amount not less than about 8%, into green balls suitable for charging into steelmaking furnaces.
The use of large amounts of oxygen in steelmaking processes has led to the production of great quantities of fume and a need for expensive gas-cleaning equipment to prevent atmospheric pollution. Such fume, sometimes referred to as dust, consists of fine particles of elements and metallic and non-metallic compounds, such as iron oxide, zinc oxide, lead oxide, sulphur compounds, carbon, silicon dioxide, aluminum oxide, calcium oxide, magnesium oxide and the like. The particles range in size from about one tenth of a micron to about 25 microns in diameter and are suspended in the exhaust gases passing from oxygen steelmaking furnaces. To clean such gases and to recover the fume emitted during steelmaking operations various devices, such as wet scrubbers, electrostatic precipitators, bag houses, etc. are used. Depending upon the manner in which the gases are cleaned the collected fume is considered wet-collected or dry-collected. Although fume containing gases can be fairly effectively cleaned by such devices, the disposal of the collected fume has presented a considerable problem. Attempts have been made to recharge the fume into steelmaking furnaces and to sinter it for charging into blast furnaces but, by and large, such fume is currently being dumped. However, because of the value of certain materials, such as iron and alloys, in fume, steelmakers have continued efforts to make use of such fume and in particular to recharge it into steelmaking furnaces.
There are a variety of ways in which particulate materials, such as iron ore concentrates, fume and the like may be caused to cohere or form into generally sperical agglomerates. They may be nodulized, briquetted, sintered, pelletized or balled. The balling process is particularly desirable for the agglomeration of finely divided particles, such as fume, because they are normally of such size that they will form into balls of relatively uniform size with little difficulty. The size of the balls formed on a balling device is directly related to the speed of rotation of the balling device, its inclination, the amount of particulate material held on the balling device during the balling operation, the feed rate of particulate material to the device, and the residence time of the particulate material on such device. The physical strength of the balls discharged from the balling device is dependent upon the size distribution of the particles of the material, the packing characteristics of the particulate material, the cohesive forces which are present when the particulate material is moist, the amount of moisture present in the balls and their porosity.
The balls, which are formed on a rotating balling device, such as a disc, cone, or drum, may be used as green balls or may be heat treated at elevated temperatures to produce heat-hardened pellets. Heat-hardened pellets have substantially greater strength than green balls and can be successfully charged into steelmaking furnaces. However, because of the cost of the heating step steelmakers would prefer not to have to resort to the heat-hardening treatment for such balls. Strong and stable green balls made from wet-collected fume and mixtures of wet-collected and water-leached dry-collected fume have also been successfully charged into steelmaking furnaces. However, heretofore, steelmakers have not been able to form dry-collected fume into green balls which could be successfully charged into steelmaking furnaces. Invariably, after dry-collected fume has been formed into green balls they have disintegrated after being placed in storage for only a short period of time.
The term green balls as used herein refers to balled material which has not been hardened by heating, i.e., after balling there is no heating to produce a sintered or heat-bonded ball. Obviously for a green ball to be used satisfactorily in steelmaking operations it must have sufficient strength to support a load and to resist degradation during transport and handling. Hereinafter strong and stable will be used to describe green balls which have met a "strength test", which is one test that has been utilized in the balling and pelletizing industry to determine whether or not a ball or pellet will survive handling in service. To satisfactorily meet the requirements of the strength test a green ball must have a compression strength of at least 15 pounds per square inch and be able to survive at least 10 drops from a height of 18 in. onto a steel plate. It has been found that strong and stable green balls, i.e., balls which have passed the strength test, will successfully resist degradation during transport and handling and will be suitable for charging into steelmaking furnaces.